It has been postulated that steam or gas and surfactant coact with liquid water and formation fluids to form foam which tends to block highly permeable channels that may allow "fingering" or "gravity override" of the steam through the formation. In a mature steam drive, residual oil saturations (S.sub.or) are frequently less than 15% in the highly permeable steam override zones or channels. In these circumstances, it is desirable to divert the steam from the oil-depleted, high permeability channels into the less permeable zones having high oil saturations. The best foaming agent for these cases foams in the oil depleted channels but does not foam and block access to the zones having high oil saturations. Examples of surfactants with these properties are provided in U.S. Pat. No. 4,556,107, which surfactants can be very effective for diverting steam from oil depleted channels into zones with high oil saturations as long as conditions are suitable for generating a foam in the oil-depleted high permeability channels. It is beneficial for said foams to be very oil sensitive, so that foaming does not occur where oil saturations are high and block steam access to the high oil zones. However, this same beneficial oil sensitivity can be a disadvantage when pockets or localized areas of high oil saturations are present within the generally oil-depleted, high permeability channels, because those pockets or localized areas of high oil can interfere with foam generation and even prevent the development of the steam diverting foam.
It is an objective of this invention to provide a process which helps assure diversion of steam from the high permeability channels into zones having higher oil saturation, even when localized pockets of high oil saturations occur in the high permeability channels.
Steam stimulation of petroleum-bearing formations, or reservoirs, has become one of the preferred methods of enhanced oil recovery. This is because steam is a cost-effective means to supply heat to low-gravity, high viscosity oils. Heat reduces resistance of oil flow from a reservoir to a producing well over a wide range of formation permeabilities. Further, such steam injection enhances the natural reservoir pressure, above that due to the hydrostatic head, or depth-pressure gradient, to increase the differential pressure between oil in the reservoir and the producing well bore.
The producing well may be the same well through which steam is periodically injected to stimulate petroleum flow from the reservoir (popularly called "huff and puff"). Alternatively, one or more producing wells may be spaced from the injection well so that the injected steam drives petroleum through the reservoir to at least one such producing well.
Almost all earth formations which form petroleum reservoirs are created by sedimentary deposition, with subsequent compaction or crystallization of the rock matrix. Such deposition of detrital materials, with varying composition and over extensive geological times, occurs at varying rates. The resulting compacted rocks in which petroleum accumulates are permeable, but in general the flow paths are quite heterogeneous. Accordingly, a petroleum reservoir formed by such rock formations is inherently inhomogeneous as to both porosity and permeability for fluid flow of either native (connate) or injected fluids. Furthermore, flow permeability for connate gas, oil and water is substantially different for each liquid or mixture. Because of these differences in permeability, it is common practice to inject foam forming surfactants with the injected steam to block the more permeable gas passages that may develop in the formation. The desired result is to divert steam from the more permeable gas passageway to less permeable oil-rich zones of the reservoir. The foaming component is usually an organic surfactant material.
This invention is an improvement over prior methods of using foam-forming compositions to enhance petroleum production from oil-bearing formations. A number of these prior methods are mentioned and discussed in U.S. Pat. Nos. 4,086,964, 4,393,937, 4,532,993 and 4,161,217.
The need for surfactants which foam in the presence of both oil and water has been known for some time. Bernard ("Effect of Foam on Recovery of Oil by Gas Drive", Production Monthly, 27 No. 1, 18-21, 1963) noted that the best foaming surfactants for immiscible displacements such as steam floods are those which foam when both oil and water are present. However, Duerksen, et al. in U.S. Pat. No. 4,556,107 recognized the advantage of using a selective foaming agent which functions as a steam diverter, foaming in the oil depleted zones but not in the high oil saturation zones where the foam would block access of the steam to the oil. Suitable surfactants for foaming in the presence of both oil and water are the branched alkyl aromatic sulfonate surfactants described in copending application U.S. Ser. No. 07/055,148, filed May 28, 1987, now abandoned. The alpha-olefin sulfonate dimer (AOSD) surfactants of U.S. Pat. No. 4,556,107 are also suitable selective foaming agents for providing steam diversion. Typically these two types of surfactants are used under different circumstances. The steam diversion surfactants of U.S. Pat. No. 4,556,107 are used to counteract channeling and override where oil saturations in the high permeability channels are typically less than about 15% of the available pore space. These conditions are usually encountered in mature steam floods where the channels have been steamed to low oil saturations. The oil-tolerant surfactants of U.S. Ser. No. 07/055,148 are used for improving oil recovery from steam floods where the oil saturations in the channels are approximately 15% or higher. These conditions can occur in young steam floods or in channels which can be resaturated with oil by gravity drainage.
The present invention provides a process for achieving efficient steam diversion over a wide range of oil saturation levels. The process of this invention overcomes the disadvantages of the oil-sensitive surfactants, such as the alpha-olefin sulfonate dimers of U.S. Pat. No. 4,556,107, without sacrificing the efficient steam diversion properties these surfactants provide. This invention, therefore, provides a means to enhance the performance of the alpha-olefin sulfonate dimers in enhanced oil recovery operations. This invention also makes it unnecessary to use separate oil-tolerant surfactants.
The above-mentioned patents and applications are incorporated herein by reference.